Semiconductor integrated circuits include many devices formed on a semiconductor body, such as a substrate. These devices, such as transistors, are formed in active areas in the semiconductor body. The active areas are typically isolated from one another by insulating regions. For example, for semiconductor devices formed with dimensions greater than 0.5 μm typically use field oxide isolation, known as LOCOS. Smaller dimension integrated circuits, such as those of 0.25 μm and below, typically use shallow trench isolation (STI).
FIGS. 1a–1c illustrate a typical process for forming shallow trench isolated active areas 20. As shown in FIG. 1a, a semiconductor substrate 10 is provided. A masking layer 12, typically a combination of silicon nitride and oxide layers, is formed over the substrate 10. Openings 14 are formed in the masking layer 12 in the areas where the isolation will be formed. While not shown, standard photoresist lithography (e.g., using a hardmask for 90 nm and below) is used.
Referring now to FIG. 1b, trenches 16 are etched using the masking layer 12 as a mask. The portions 20 of the semiconductor substrate 10 adjacent the trenches 16 will be the active areas. Transistors and other devices can be formed in the active areas 20. As shown in FIG. 1c, the trenches 16 are filled with an insulating material in order to isolate the devices in the active areas from one another.
One issue for shallow trench isolation is the trench filling. A high density plasma (HDP) is widely used for this process. Many process conditions have been tried for optimization. As the top trench critical dimension (CD) is getting smaller, however, the HDP deposition is becoming very difficult due to voids that are formed. These issues only get worse as the CD goes from 90 nm and smaller and as the STI depth gets deeper for better isolation. A typical measurement is the aspect ratio (AR), which is defined as STI depth over CD width. Conventional wisdom is that oxide filling is getting more difficult as the AR goes above 3.
To avoid these problems, the main focus has been directed to modifying the process conditions and the combination of deposition and etch back. In other processes, low K dielectrics have been used due to their better reflow characteristics. The thinking is that better reflow characteristics will lead to better fill characteristics. Unfortunately, low k dielectrics have problems of their own.
Another problem with STI is that it involves many process steps: one masking step, one RIE, a liner oxide, fill, CMP and many cleans. A greater number of process steps leads to a higher cost. Therefore, a need has arisen for a lower cost process that avoids the problems of shallow trench isolation.